I just remember my DSP teacher told me that the digital signal sample rate mean nothing in digital world, but in the "real" word it is important and sample rate is we assign the meaning to a group of data and we can to like fft or something needs time. So it's my fault. Thank you for remind me zero padding, and I "think" I can create a block like source, because like signal source can set the sample rate and I also create a block can set sampling rate and give it a function that accept packet. Also I can try bursts data, I never done before. Using constant 0 source I think I doesn't work because after add message-generated the output data also same as message-generated.
If I just connect message-generated to sink, it worked well. This problem will show up when discrete source and continuous source at a same time and trying to combine them together.
This question is from my project, but it's finish. My solution is let the message-generated floating and when I need to send packet I lock the flow graph and reconfigure the connect and unlock, after sending the packet also reconfigure them.
Recently, I want to do not only physical layer but data link layer or upper layer and this problem just come to my mind. If I using gnu radio like ethernet it work well because usurp sink will send data when it get them. But I want to create a system, so I cannot avoid to facing this problem. Thanks for answering my question, help me a lot.